Storm Forecast
Valid: Mon 17 Nov 2014 06:00 to Tue 18 Nov 2014 06:00 UTC
Issued: Mon 17 Nov 2014 02:48
Forecaster: VAN DER VELDE
A level 1 was issued for Corsica, parts of Italy and western Balkan mainly for excessive convective precipitation and tornadoes.
SYNOPSIS
While high pressure reigns over Russia and Scandinavia, a large upper low controls and area from southern UK to Albania. Surface low pressure centers will be situated in the English Channel, the other moving south of France over Italy to Austria. A cold front moves across the Pyrenees during the early morning and should trigger widespread convection as steep mid level lapse rates meet moist low-level Mediterranean air. Several hundred J/kg of MLCAPE would be able to develop. The strongest lifting is at the cold front which is backed by ascent from advection of mid altitude PV and a 60 m/s jet streak at 300 hPa.
DISCUSSION
Development of generally 100-200 mē/sē (300-500 predicted over northeast Adriatic coast) and 15-30 m/s deep layer shear aids in the generation of updraft rotation, favoring highly persistent convective cells and large hail chances. Most prominent convection should develop at the cold front and leading edge of the low dynamic tropopause bubble. The front should affect Corsica from early afternoon and Italy mid-afternoon till the end of this forecast validity period.
Storms will be structured in the form of a line or individual cells surfing the moist air lifting over mountain ranges over Corsica and Italy, as well as the Balkan coasts. The slow speed and backbuilding can lead to local excessive rain accumulations. The CAPE and moisture content are not so high and frontal movement not so slow, so a level 2 was not chosen for these reasons. The strong wind(shear) environment rather poses threats of tornadoes and isolated severe wind gusts >25 m/s. Very low LCL and 0-1 km shear of 10-17 m/s over central Italy and northwestern Balkan countries should assist in this threat. Non-mesocyclonic tornadoes (waterspouts) are expected in the Ligurian Sea and northern Mediterranean, Adriatic Sea, as well as the English Channel where convective cloud tops are not reaching very cold/high levels.
